1. Field
The invention is in the field of methods and apparatus for determining the remaining charge or useful life (state-of-charge) of an electrochemical battery.
2. State of the Art
Primary electric batteries of different kinds are used for generation of electricity through conversion of chemical energy, from the reaction of their electrodes with the electrolyte solution, into electric current with a certain voltage. Once the reactive mass of the electrodes or the electrolyte is depleted, these batteries are finished and must be replaced. These primary batteries are usually manufactured for supplying limited amounts of electricity and are used in a wide range of equipment. Secondary batteries that can be recharged after depletion are capable of delivering considerable amounts of electricity with high currents and are used in a wide variety of applications, such as backup power supplies, or as the main source of power in some electric vehicles. In these batteries, the chemical energy from the reactions between the electrodes and the electrolyte that is spent on producing electricity, can be replenished by reversing the chemical reactions on the electrodes during a recharging process that involves passing an electric current, from another source, through the battery in the reverse direction. The amount of electric energy that can be extracted from a battery depends on its electric potential, expressed in Volts, and the number of Coulombs or the electric current intensity times its duration, expressed in Ampere-hours, that the battery can sustain.
A common feature of most electric batteries is their relatively constant voltage during use. The voltage is not usually dependent on the battery's charge content so an instantaneous measurement of a battery voltage does not relate to the quantity of charge remaining in the battery. Only when the battery gets close to being completely discharged does its voltage start to drop off noticeably. In other words, the battery voltage is independent of its energy content. This is particularly true in some lithium batteries. Although this is a very desirable feature in almost all cases of battery applications, it causes a problem when it comes to indicating the amount of charge that is left in a battery. It is desirable in many cases, and in some situations it is essential, to know exactly how much energy is available in a battery system at any instant during its use. A special situation of this kind is when a battery is used for powering an electric vehicle. It is obvious that the operator of the vehicle has to know the energy content of his battery at all times for planning his travel duration and for a timely renewal of its energy content. In some military situations, a correct estimate of the remaining electric charge in a battery system may be critically important to the mission of the system that runs on that battery. For these reasons, the subject of indicating the remaining charge in electric batteries has received the attention of many inventors and there are a large number of patents dealing with "state-of-charge" indicators for batteries. Examples are U.S. Pat. Nos. 3,484,681, 3,617,850, 3,895,284, 3,898,548, 3,906,329, 4,307,330, 5,315,228, and 5,315,253.
Most of the devices described in the prior art patents measure the electrical signals of a battery cell or measure charging current into a battery and current taken from the battery and, using them in a variety of procedures, estimate how much electric charge is left in the battery at any moment. Examples include measurements of voltage and time-integrated currents that pass through the battery, in charging and discharging situations, along with temperature measurements for applying necessary correction factors. A common disadvantage of most of these patented methods and devices is the necessity of continuous monitoring and electronic book-keeping of the past current history of the battery, and none of them is a truly instantaneous charge indicator.
A different approach is shown by U.S. Pat. No. 4,129,824 which measures the specific gravity of the acid solution in a lead acid storage battery and equates the specific gravity measured to the charge left in the battery. This approach assumes that the specific gravity of the acid solution is an accurate measure of the charge and does not take into account dilution of the solution, evaporation of the solution, or disassociation of the solution which occurs during charging, all of which have an effect on the density of the solution unrelated to the charge remaining in the battery. Further, this methods applies only to lead-acid batteries and it is not suggested as universally applicable to a wide variety of types of batteries.
There remains a need for an accurate system for measuring the charge in a battery that can be applied universally to and used with a variety of types of batteries.